The scale on a map is 2cm : 8km. If the distance between two cities is 40km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 2cm on the map corresponds to an actual distance of 8km. An actual distance of 40km is the same as 5 $\cdot$ 8km. The distance between the two cities on the map is 5 $\cdot$ 2cm, or 10cm.